Field of the Invention
This disclosure generally relates to fiber based optical parametric oscillators.
Description of Related Art
Light sources based on optical parametric interaction are interesting since they provide access to laser wavelengths that existing gain materials based on electronic transitions cannot provide. An optical parametric oscillator (OPO) can be realized by exploiting the χ(2) nonlinear optical response in a wide range of crystals or the χ(3) nonlinear response in optical fibers.
Optical fiber based OPO (FOPO) are particularly attractive owning to their potential in achieving low cost, alignment-free and compact laser systems while still providing very wide tuning range and high power operation.
The operation of FOPOs is in essence based on degenerated four-wave-mixing (FWM) wherein two pump photons interact with the fiber to generate a signal photon and an idler photon. The exact frequencies of the signal and idler photons are defined by the phase matching condition which depends on the pump laser wavelength, its peak power as well as the dispersion profile of the optical fiber of the FOPO. There are two common ways to pump an OPO. The first approach is continuous pumping where the pump laser is a continuous wave laser or a laser generating long pulses compared to the OPO cavity round trip time. The second approach is based on synchronous pumping of pulsed pump laser. In the context of the present application, the optical frequency of signal photon may be greater than the optical frequency of the idler photon; or the optical frequency of signal photon may be less than the optical frequency of the idler photon. This has been done to simplify the explanation, a resonating ring cavity may be designed to operate to produce either a signal pulse or an idler pulse, while the description below has been written in terms of a signal pulse, but an embodiment may be made to operate to output either a signal pulse, an idler pulse, or both.
Pulsed lasers tend to have a broader output optical spectrum than continuous lasers. One way to narrow the spectral bandwidth of a pulsed laser is with the use of a spectral filter, preferably tunable, that is outside of the laser cavity. In which case, the spectral bandwidth of the pulsed laser is influenced mainly by the spectral shape of the spectral filter. Since, the spectral shape of the pump light is tuned with a wavelength tunable external filter; this makes it difficult for a narrow spectral bandwidth to be achieved. This is because the spectral bandwidth of the pump light is almost same as the spectral bandwidth of the tunable external filter. In which case, the conversion efficiency from pump light to signal light is drastically decreased due to the broad spectral bandwidth of the pump light. As a result, it is difficult to achieve a high peak power of the signal light.